1. Technical Field
The present invention relates generally to heat transfer devices, and more specifically to heat pipes having wick structures.
2. Related Art
A typical heat pipe transports heat in the form of latent heat of a working fluid thereof. The heat pipe is made of a heat conducting material. In assembly, air is evacuated from the heat pipe, a condensable fluid such as water is filled in the heat pipe, and then the heat pipe is sealed. The heat pipe is essentially a receptacle (container) which transports heat as latent heat of the working fluid therein. Heat input from outside the heat pipe evaporates the working fluid, the vapor flows to a condenser section of the heat pipe having a low temperature and a low pressure, the vapor condenses, and the released heat radiates from the condenser section of the heat pipe. Because the heat is transmitted in the form of latent heat of the working fluid, the heat pipe has from more than ten times to several hundred times the heat transmitting capacity of that of copper, which is generally considered to have the highest heat conductivity among common metals.
The evaporated vapor phase working fluid flows to the condenser section due to the temperature and pressure differentials. After the heat is released, in a typical heat pipe, the condensed liquid phase working fluid is refluxed to the evaporator section by capillary action of a wick contained within the heat pipe.
Referring to FIGS. 4 and 5, a conventional heat pipe 10 includes a pipe 11, a wick 12 formed on an inner wall 17 of the pipe 11, and a working fluid 13. The working fluid 13 is sealed in the pipe 11 and soaked in the wick 12. Before filling the wick 12 with the working fluid 13 and sealing the pipe 11, the inside of the pipe 11 has to be evacuated. The pipe 11 has two end sections. One of the end sections is an evaporator section 10a (a heating section). The other end section is a condenser section 10b (a cooling section). An adiabatic section may be provided between the evaporator and condenser sections, according to practical need. A vapor 14 of the working fluid 13 in the wick 12 is generated by the working fluid 13 being heated at the evaporator section 10a of the heat pipe 10. The vapor 14 flows through a hole 16 to the condenser section 10b where the temperature and the pressure are low. Then, the heat 15 is dispersed from the vapor 14 at the condenser section 10b, and the vapor 14 is condensed and liquefied back to working fluid 13. After that, the working fluid 13 is refluxed to the evaporator section 10a by capillary action of the wick 12. According to this system, the heat 15 is taken continuously from the evaporator section 10a of the heat pipe 10 to the condenser section 10b of the heat pipe 10, and is dissipated at the condenser section 10b. 
The wick is essentially a member for creating capillary pressure. Therefore, it is preferable that the wick 12 has excellent so-called hydrophilicity with the working fluid. Further, it is preferable that the wick has an effective radius of a capillary tube as small as possible at a meniscus of the liquid phase working fluid. In this connection, a porous sintered compact or a bundle of extremely thin wires is generally employed as the wick. The porous sintered compact may create great capillary pressure, i.e. pumping force acting on the liquid phase working fluid, because the dimensions of the openings of its cavities are smaller than those of other wicking structures such as thin wires. Further, the porous sintered compact can be formed into a seat shape, so that it is easily employed in the vapor chamber of a flat plate type heat pipe or the like. Accordingly, the porous sintered compact is viewed by many as the preferred wicking material.
A sintered wick made from sintered metal powders and sintered ceramic powders is disclosed in U.S. Pat. No. 4,274,479. A heat pipe capillary wick constructed from a sintered metal cylinder is formed in close contact with an inner wall of a heat pipe casing. Longitudinal grooves are defined in an inner surface of the wick, adjacent to a vapor space of the heat pipe casing. The grooves provide longitudinal capillary pumping, while the high capillary pressure of the sintered wick provides liquid to fill the grooves. This structure assures effective circumferential distribution of liquid in the heat pipe. However, because of the large particle size of the powder and small evaporator surface area of the sintered wick, the wick has relatively high thermal resistance and low heat transfer capacity. In particular, because only one axis hole is defined within the heat pipe casing, the sintered wick has only a single inner circumference communicating with the vapor space. This provides only a relatively small evaporator surface area.
What is needed, therefore, is a heat pipe with low thermal resistance, a high evaporator surface area, high capillary force, and good heat transfer capability.